Process for removing sox and nox compounds from gas streams

ABSTRACT

A process for removing SO x  pollutants from a stack gas by (1) absorbing the SO x  pollutants into an aqueous absorbent containing a formate compound and (2) regenerating the spent absorbent containing dissolved SO x  compounds by contact, in the presence of added formate anion, with a water-insoluble, solid substance containing one or more tertiary amine functional groups. Nitrogen monoxide is removed by providing in the aqueous absorbent an iron(II) chelate, such as a chelate of ferrous ion with ethylenediaminetetraacetic acid. Regeneration of the spent absorbent containing absorbed NO is accomplished under the same conditions as for spent absorbents containing absorbed SO x  compounds. SO x  and NO pollutants dissolved in the absorbent are, during regeneration, converted to hydrogen sulfide and nitrogen, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a division of copending U.S. patentapplication Ser. No. 151,783 filed May 21, 1980, now U.S. Pat. No.4,372,932, which is itself a continuation in part of copendingapplication Ser. No. 907,189 filed May 18, 1978, now U.S. Pat. No.4,222,991.

BACKGROUND OF THE INVENTION

This invention relates to the removal of SO_(x) and NO_(x) compoundsfrom waste gas streams. More particularly, the invention relates to theremoval of SO₂, SO₃, NO, and NO₂ from industrial stack gases.

The burning of coal or oil as fuel in a boiler or furnace produces aflue gas (or stack gas) usually containing SO₂, SO₃, and NO_(x). Theconcentration of these components in a particular stack gas depends uponsuch factors as the concentration of sulfur and nitrogen components inthe fuel, the metals concentration in the fuel, the air rate fed to theboiler or furnace, and the temperature of combustion. A stack gas,however, will usually contain between about 10 and 2000 ppmv NO_(x) andabout 100 ppmv and 5 mole percent SO_(x) compounds, with the largemajority of the latter, usually at least about 95% thereof, being in theform of SO₂.

Before a stack gas containing SO_(x) compounds may be discharged to theatmosphere, many environmental regulatory agencies require that thestack gas be desulfurized, that is, that the concentration of sulfurcompounds therein be reduced to specified levels. Similar regulationsrequire the removal of NO_(x) components in stack gas, largely for thereason that such components contribute to photochemical smog.

The most conventional method presently utilized to remove SO_(x)compounds from a stack gas involves contacting the stack gas with aliquid absorbent containing dissolved lime or caustic. But although sucha process is favorable from a cost standpoint, it is largely ineffectivefor removing NO, which might also be present in the stack gas. Inaddition, since the spent absorbent obtained from a lime or caustictreatment is regenerable only by extremely costly techniques, the spentabsorbent is usually not regenerated; instead, it is allowed toaccumulate for waste disposal, which requires in many instances thedaily handling of tons of spent absorbent. Thus, waste disposal of spentabsorbent is a nuisance at the least and oftentimes a very difficulttask.

Accordingly, it is an object of this invention to provide a process forremoving SO_(x) from a gas stream while minimizing the amount of wasteproducts produced. It is a further object to provide a process for (1)simultaneously and effectively removing both SO_(x) and NO_(x)components from a gas stream by absorption in an aqueous liquid and (2)easily regenerating the aqueous liquid when laden with dissolved SO_(x)and NO_(x) compounds such that said SO_(x) and NO_(x) compounds areconverted to hydrogen sulfide and nitrogen. Other objects and advantagesof the invention will be apparent in view of the following descriptionof the invention.

SUMMARY OF THE INVENTION

According to this invention, SO_(x) compounds present in stack gases andother feed gases are removed therefrom by contact with an absorbentcomprising an aqueous solution of one or more formate compounds. Theprocess is most successfully accomplished in an absorption zone intowhich the feed gas and fresh (or regenerated) absorbent are introducedand from which a desulfurized product gas and spent absorbent arecontinuously removed.

Provision is also made in the invention for regenerating the spentabsorbent to a form once again active for removing SO_(x) compounds.This is accomplished by contacting the spent absorbent, in the presenceof added formate ion, and under conditions of elevated temperature andpressure, with a water-insoluble, solid substance containing a tertiaryamine functional group. Such contacting results in the regeneration ofthe absorbent by converting a substantial proportion of the dissolvedsulfur constituents to hydrogen sulfide, which hydrogen sulfide isremoved with other gases from the regenerated, aqueous absorbent byseparation in either the regeneration zone itself or in a suitablegas-liquid separator. Once separated from non-condensable gases, theregenerated absorbent is in a condition for recycle to the absorptionzone.

In one alternative embodiment of the invention, a water-soluble iron(II)chelate is introduced into the absorbent for the purpose of absorbing NOwhich might also be present along with the SO_(x) compounds in the feedgas stream. In such embodiment, regeneration of the spent absorbent inthe manner hereinbefore described results in the absorbent becomingactive for the absorption of both SO_(x) and NO compounds.

In yet another embodiment of the invention, wherein the stack gas isfree of SO_(x) compounds but contains NO, as might be the case if thestack gas consists of the gases produced by combusting a desulfurizedfossil fuel, the NO itself may be removed by contact in an absorptionzone with an aqueous solution containing a dissolved iron(II) chelate.Regeneration of the absorbent is accomplished in the same manner asabove described, that is, by contact in the presence of added formateion with a water insoluble, solid substance containing one or moretertiary amine functional groups.

As used herein, the term absorbent refers to the aqueous medium used inthe process to remove SO_(x) and/or NO_(x) from the feed gas, regardlessof the particular mechanism by which the SO_(x) and NO_(x) componentsare removed from the feed gas and retained by the absorbent. Also,reference to SO_(x) is meant to include SO₂ and SO₃ while reference toNO_(x) is meant to include NO and NO₂.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 of the drawing is a schematic flowsheet of the preferredembodiment of the invention for removing SO_(x) and NO_(x) pollutantsfrom a feed gas. For simplicity, devices such as pressure relief valves,back pressure regulators, and other conventional equipment have not beenrepresented in the drawing.

FIG. 2 of the drawing is a graphic representation of the relationship ofthe operating conditions during regeneration to the conversion ofdissolved SO₂ in the absorbent to sulfur and hydrogen sulfide.

DETAILED DESCRIPTION OF THE INVENTION

The gas streams suitable for treatment in the process of this inventionare those gas streams containing SO_(x) and/or NO_(x) compounds.Preferred gas streams for treatment herein contain at least some SO_(x)compounds, most preferably in concentrations exceeding 500 ppmv, with asubstantial proportion, usually above 95%, of the SO_(x) componentsconsisting of SO₂. Such gas streams include waste gas streams, such as aClaus tail gas stream that has been subjected to oxidation so as tocontain SO₂ as the major sulfur component. Also included are waste gasesproduced by roasting sulfided ores. The preferred waste gas stream,however, is a particulate-free stack gas containing SO₂ and SO₃ asessentially the only sulfur compounds therein. A typical stack gasobtained by the combustion of a sulfur-containing fossil fuel containsthe several gaseous components listed in Table I in the range ofconcentrations shown.

                  TABLE 1                                                         ______________________________________                                        Component Mol %       Component ppmv                                          ______________________________________                                        O.sub.2   1-5         CO        0-500                                         CO.sub.2  10-20       NO.sub.x  10-2000                                       H.sub.2 O  5-25       SO.sub.2   100-50,000                                   N.sub.2   70-75       SO.sub.3  0-200                                         ______________________________________                                    

The method of the invention will now be described in detail, withattention first being directed to removing SO_(x) and then to removingNO_(x).

SO_(x) REMOVAL

Referring now to the drawing, the preferred embodiment of the inventionwill be described with relation to removing SO_(x) components from aparticulate-free stack gas containing SO₂ and SO₃ in concentrationsfalling in the typical ranges listed in Table I. The stack gas is fed byline 1 at a convenient temperature, usually less than about 200° F., andat a rate between about 1000 SCF/hr and about 100,000 SCF/hr and at apressure slightly above atmospheric but preferably less than about 15psig into absorber 2. The absorber may comprise such suitable gas-liquidabsorption equipment as a packed tower, a multi-plate column, or aventuri scrubber, but the design should be such that sufficient contacttime is provided for the SO_(x) components to react as fully as possiblewith the fresh absorbent introduced through line 3 and make-up line 23.Preferably, the absorber is of a packed tower design, and the stack gasis passed countercurrently to the flow of the absorbent. An essentiallySO_(x) -free and desulfurized product gas is thus discharged to theatmosphere by line 4 while spent absorbent containing dissolved SO_(x)compounds is withdrawn via conduit 5.

The fresh absorbent fed into absorber 2 via lines 3 and 23 is an aqueoussolution containing one or more water-soluble formate compounds, such assodium formate, lithium formate, potassium formate, ammonium formate, orformic acid. The preferred aqueous absorbent comprises sodium formate,especially when buffered with formic acid to a pH in the 2.4-5.0 range.

Optionally, but not preferably, the fresh absorbent fed into the systemvia line 23 contains an alkaline agent in addition to the formatecompound or compounds. The alkaline agent, if used, is preferably sodiumhydroxide, but ammonium hydroxide or any water-soluble metal hydroxide,especially the alkali metal hydroxides, may be used, as also may suchcomponents as sodium carbonate, sodium bicarbonate, and otherwater-soluble salts of a strong base and a weak acid. For treating atypical stack gas of composition shown in Table I, the fresh absorbentadded via line 3 typically contains 5-30 wt.% of sodium formate, 0-20wt.% of additional alkaline agent, and sufficient formic acid tomaintain the pH of the absorbent entering the absorber between about 2.5and 10.0, and more preferably between 3.5 and 5.0. A preferredcomposition comprises 5 to 10 wt.% sodium formate and sufficient formicacid to maintain the pH of the fresh absorbent in line 3 in the 3.5-5.0range. A still more preferred absorbent composition comprises 6.2 wt.%sodium formate and sufficient formic acid to buffer the composition at apH of 4.0.

Although the invention is not intended to be limited to any particulartheory of operation, it is believed that the chemical reactions in theabsorber between the gaseous components in stack gas of compositionshown in Table I and the components of the preferred aqueous absorbentcontaining no alkaline agent and having an acidic pH include thefollowing: ##STR1##

When the absorber is operated under preferred conditions, the flow rateof the absorbent fed via line 3 is so correlated with the amount ofSO_(x) removed from the stack gas that spent absorbent is withdrawn fromabsorber 2 via conduit 5 at a pH less than 7.0, preferably less than5.0, thereby avoiding withdrawal of a solution rich in dissolved CO₂from the stack gas. When the preferred absorbent containing noadditional alkaline agent is utilized, the pH of the spent absorbent iswithdrawn at a buffered pH between about 3.0 and 4.5, and morepreferably still, between 3.5 and 4.3.

The spent absorbent in line 5 must be regenerated before being recycledto absorber 2 through line 3. However, in order to regenerate the spentabsorbent in accordance with the invention, it is first heated to anelevated temperature, usually between about 240° and 350° F., andpreferably between 270° and 290° F., and most preferably to atemperature of 285° F. Thus, the absorbent in line 5 is directed by pump6 through pipe 7 to heat exchanger 8 wherein by indirect heat exchangewith heated, regenerated absorbent the temperature of the spentabsorbent is raised to about 150°-200° F. The absorbent is thenintroduced by conduit 9 into a second preheater 10, wherefrom heatedabsorbent is obtained in line 11 at a temperature of at least 240° F.,preferably at a temperature of about 270° to 290° F. To the heatedabsorbent is added an aqueous solution containing a dissolved formatecompound, such as dissolved formic acid, through line 13 and valve 14.The resulting mixture in pipe 12 is fed into regenerator 15.

In regenerator 15, the spent absorbent solution, now containing addedformate ion from the formic acid, is regenerated back to its active formby contact at a pressure at least sufficient to prevent the absorbentfrom boiling and at a temperature of 240° to 350° F. with awater-insoluble, solid substance containing one or more tertiary amine(a valence state 3 nitrogen atom bonded only to carbon atoms) functionalgroups, with said functional groups preferably having anion exchangecapacity and being ion exchanged to contain formate ion in at least someof the ion exchange sites. Preferably, a water-insoluble, anionic, ionexchange resin containing tertiary amine functional groups, such as thatmarketed by Rohm & Haas Company under the trade name of Amberlyst A-21is utilized. Although the exact composition of this resin is notavailable to the public, it is known that Amberlyst A-21 is awater-insoluble organic resin of macroreticular structure comprising acopolymer of a substituted styrene and divinylbenzene and containingweakly basic, tertiary amine functional groups, with essentially all ofthe anion exchange capacity residing in said tertiary amine functionalgroups. It is further known that Amberlyst A-21 is stable in regenerator15 at the preferred operating temperature of 285° F., and that, whentreated to contain formate ions at the ion exchange sites, it hascatalytic activity for regenerating the spent absorbent to a form activefor absorbing SO_(x) compounds. Hence, when initially charged toregenerator 15, the Amberlyst A-21 resin particles are preferablyion-exchanged to contain the formate ion by pretreatment with hot formicacid under an inert or reducing atmosphere. In non-preferredembodiments, however, the resin may be pretreated by ion-exchange with asalt of formic acid, such as sodium formate, or the resin may simply beconverted to the active form in situ, e.g., by adding sufficient formicacid via line 13 to activate the resin during regeneration. In allembodiments, however, it is preferred that formate ions occupy at least50%, and preferably essentially all, of the anionic ion exchange sites.

In choosing a resin for use in regenerator 15, it is most highlypreferred that the resin contain essentially no primary or secondaryamines, especially if such primary or secondary amines contribute to theanion exchange capacity of the resin. It is believed that Amberlyst A-21resin contains essentially no primary or secondary amines and that oneof the reasons it is highly active for the chemical reactions requiredin regenerator 15 is that it is free or essentially free of primary andsecondary amines. The number of quaternary amines in the resin shouldalso be relatively low due to their strongly basic characteristics.Amberlyst A-21 has some quaternary amines but not enough to alter theweakly basic characteristics of the resin.

As an alternative to Amberlyst A-21 resin, it is a specific embodimentof the invention to utilize an organic resin comprising a copolymer ofdivinylbenzene, styrene, and a vinylpyridine, such as 2-vinylpyridine or4-vinylpyridine, or a substituted vinylpyridine, such as2-methyl-5-vinylpyridine. A method by which one such organic resin isprepared is by heating at 50° C. for one day a mixture composed of 75parts by weight 2-methyl-5-vinylpyridine, 22 parts styrene, 3 partsdivinylbenzene, 5 parts of a sodium fatty acid soap, 180 parts water(deionized), 0.3 parts by weight mixed tert-mercaptans, and 0.3 parts byweight potassium persulfate, following which the copolymer product isseparated by conventional means from unreacted ingredients and dried.Another method is similar to the foregoing except that 75 parts of4-vinylpyridine are utilized instead of 2-methyl-5-vinylpyridine and 0.3parts of benzoyl peroxide is utilized in place of the potassiumpersulfate.

Although the invention is not limited to any particular theory ofoperation, it is believed that, when formic acid is the formate compoundadded via line 13, the chemical reactions resulting in the conversion ofthe absorbent solution back to its active form include the followingionic reactions:

    2H.sup.+ +SO.sub.3.sup.= +3HCO.sub.2 H→H.sub.2 S+3CO.sub.2 +3H.sub.2 O                                                         (VIII)

    H.sup.+ +HSO.sub.3.sup.- +3HCO.sub.2 H→H.sub.2 S+3CO.sub.2 +3H.sub.2 O                                                         (IX)

    2H.sup.+ +S.sub.2 O.sub.3.sup.= +4HCO.sub.2 H→2H.sub.2 S+4CO.sub.2 +3H.sub.2 O                                               (X)

It is also possible that some formic acid may be decomposed inregenerator 15 by reaction with dissolved oxygen as follows:

    O.sub.2(aq) +2HCO.sub.2 H→2CO.sub.2 +2H.sub.2 O     (XI)

Regeneration of the spent absorbent in regenerator 15 may beconveniently accomplished in a stirred reactor vessel wherein theabsorbent and the organic resin are slurried for a time sufficient toconvert a substantial proportion of the dissolved sulfur components toH₂ S. Alternatively, the regeneration may be accomplished in a fixed bedreactor in which the spent absorbent must pass through a bed ofAmberlyst A-21 resin (or other suitable resin) maintained to a depth ofat least two feet, more preferably at least three feet, with thepreferred ratio of bed depth to average bed diameter being 5:1. When afixed bed reactor is used, the absorbent is passed through the bed ofresin at a liquid hourly space velocity of at least 0.1, preferably atleast 0.3, but no more than 10.0. The preferred operating conditionsmaintained in a fixed bed or stirred reactor are as follows: 285° F., 63psia, 0.4 LHSV.

When preferred conditions are utilized in regenerator 15, the effluentin line 16 will be a two-phase fluid comprising a non-condensable gasphase and a liquid phase of regenerated absorbent. This effluent isdirected to a gas-liquid separator 18, where the regenerated absorbentat a pH between about 2.5 and 10.0, preferably 3.5 to 5.0, is separatedfrom the non-condensable gases and withdrawn through line 19. Thenon-condensable gases discharged through vent valve 17 contain a largeproportion of CO₂, usually in a concentration of 60 to 90% by volume(dry basis), with the remainder of the gases comprising hydrogen,hydrogen sulfide, water vapor, nitrogen, and trace organosulfur gases.The non-condensable gases are preferably directed to sulfur recoveryfacilities (not shown in the drawing) wherein the H₂ S is converted tosulfur or SO₂, as by catalytically reacting the H₂ S with oxygen at atemperature above about 275° F., preferably above 350° F., in thepresence of a catalyst comprising vanadium oxide or vanadium oxide plusbismuth oxide on silica-alumina (or other porous refractory oxide). Moredetailed disclosures relating to the catalytic conversion of H₂ S tosulfur or SO₂ may be found in U.S. Pat No. 4,123,507, hereinincorporated by reference.

The regenerated absorbent recovered in line 19 is directed by pump 20and conduit 21 to heat exchanger 8 and thence to absorber 2 via lines 22and 3. Eventually, steady state conditions will be attained, and thefresh absorbent feed from make-up line 23 is either shut-off or reducedas required by operating control valve 25. Line 24 and control valve 26are provided to bleed absorbent from the system as required.

Under steady state conditions, the regeneration of the absorbent iscontrolled largely by the amount of formic acid added via line 13 andthe temperature maintained in regenerator 15. Formic acid is preferablyadded at a rate at least sufficient to effect full conversion ofdissolved SO₂ components (largely in the form of sulfite or bisulfiteion) to H₂ S according to:

    3HCO.sub.2 H+SO.sub.2 →H.sub.2 S+3CO.sub.2 +2H.sub.2 O (XII)

As the absorbent traverses the regeneration zone, its pH will increasewith increasing conversions of dissolved SO₂ to H₂ S. This conversion ismaximized by controlling the temperature in regenerator 15. As shown inFIG. 2, the conversion of dissolved SO₂ to H₂ S increases dramaticallyin the temperature range of 260° to 300° F., approaching its maximum attemperatures above about 280° F.

It is noted with respect to the data in FIG. 2 that the dissolved SO₂components may be converted to elemental sulfur. A process whereindissolved SO₂ is converted to elemental sulfur in regenerator 15 isdescribed in copending application Ser. No. 907,189, filed May 18, 1978,now U.S. Pat. No. 4,222,991 herein incorporated by reference. As thedata in FIG. 2 show, the resin utilized in regenerator 15 catalyzes theconversion of dissolved SO₂ components to hydrogen sulfide and/orsulfur, depending upon the temperature maintained in the regenerator.Temperatures of about 205° to 250° F. result in large yields ofelemental sulfur while temperatures of 280° of 300° F. result in largeyields of hydrogen sulfide. Temperatures of 250° to 280° F. result inthe conversion of dissolved SO₂ components to both hydrogen sulfide andsulfur, with the proportion of hydrogen sulfide increasing in theproduct gas with increasing temperature.

Under steady state conditions, the chemical composition of theregenerated absorbent in line 22 will contain not only the componentsoriginally added via make-up line 23, but also residualsulfur-containing anions, particularly sulfate anions. The concentrationof these sulfur-containing anions will increase dramatically whenconditions are maintained in regenerator 15 that do not result in highconversions of dissolved SO₂ to H₂ S or, less preferably, to sulfur orH₂ S plus sulfur. In particular, if the temperature chosen for theregenerator is so high that the resin begins to degrade, the regeneratedabsorbent in line 10 will gradually increase in concentration ofsulfur-containing anions in direct relationship to the gradual loss ofcatalytic activity of the resin. Thus, for best results, and especiallyto maintain the make-up rate of fresh absorbent in line 23 and thebleed-rate of spent absorbent in line 24 as low as possible, regenerator15 should be operated under the conditions stated hereinbefore, and mostspecifically at a preferred temperature between about 280° F. and thetemperature at which the particular resin begins to degrade.

Even under preferred conditions, however, some sulfur-containing anions,and particularly sulfate anions, will increase in concentration in thecirculating absorbent. Although extremely high conversions (usuallyexceeding 95%) of dissolved sulfite, bisulfite, and thiosulfate ions tohydrogen sulfide have been found to take place in regenerator 15, it hasalso been found that sulfate ions prove exceptionally difficult toreduce in regenerator 15, and it is usually not possible to obtain highconversions of sulfate ion (formed either by the direct dissolution ofSO₃ or by the reaction of dissolved SO₂ with dissolved oxygen) tohydrogen sulfide in regenerator 15. Thus, a bleed through line 24 willusually be necessary, not only to prevent exceeding the solubility limitof sulfate salts in the circulating absorbent but also to prevent thedisplacement of formate ions by sulfate ions from the resin inregenerator 15. It has, however, been found that a sulfate concentrationas high as 10 g/l can be tolerated in the circulating absorbent withoutencountering difficulties.

The process as described is highly efficient when preferred conditionsare utilized throughout. For a stack gas containing 2,000 vppm SO₂, thedesulfurized purified gas discharged via line 4 typically contains lessthan 200 vppm, usually less than 40 vppm, of SO₂. The desulfurizedpurified gas typically will carry, on a mass per hour basis, less than10%, preferably less than 5%, of the amount of SO₂ carried in the stackgas. Also, in the preferred embodiment, the volumetric rate at which H₂S is recovered from separator 18 will be at least 80%, usually at leastabout 90%, of the rate at which SO₂ is absorbed in absorber 2.

NO_(x) REMOVAL

Because many stack gases and other waste gas streams contain NO_(x)compounds in addition to SO_(x) compounds, the invention is accordinglyalso directed to removing NO_(x) from such gas streams. Although NO_(x)is present in stack gases largely as NO, some stack gases may contain upto 5%, perhaps as much as 10%, of the NO_(x) in the form of NO₂. NO₂ isreadily absorbed in absorber 2 due to its high solubility in aqueousmedia. It is believed in the invention that, when the absorbenttraversing absorber 2 contains dissolved sulfite ion, at least some NO₂reacts in the absorber with sulfite ion to produce elemental nitrogenand sulfate ion. Some NO₂ dissolves directly into the absorbent asnitrate or nitrite ions, which are ultimately converted in regenerator15 to nitrogen under the conditions hereinbefore specified. Nitrogenproduced by reduction of nitrate and/or nitrite ions in regenerator 15is recovered as a component of the product gas obtained through ventvalve 17.

The removal of NO is more difficult than is the removal of NO₂, but ithas been found that the absorbent, after the addtion of iron(II)chelates thereto, becomes useful for absorbing NO. Thus, to remove NOand SO_(x), the absorbent, in addition to containing a dissolved formatecompound, also contains a water-soluble iron(II) chelate, such as Fe(II)EDTA (ferrous ion chelated by ethylenediaminetetraacetic acid) andFe(II) HEDTA (ferrous ion chelated byN(hydroxyethyl)ethylenediaminetriacetic acid). Such chelates are usuallypresent in the absorbent in a concentration between about 0.001 and 1.0molar, perferably between about 0.1 and 0.25 molar. The chelate may beadded to the absorbent solution in any of a variety of ways, with itbeing most preferred to add an alkali metal salt of EDTA or HEDTA andiron formate. If the iron formate is ferrous formate, the iron(II)chelate readily forms; if ferric formate, then ferric chelate forms,which is easily reduced to the necessary ferrous chelate by passage ofthe absorbent through regenerator 15.

When the absorbent passing through absorber 2 contains iron(II) chelate,it is highly effective for removing NO, most probably by the directchemical reaction:

    NO+Fe(II)EDTA→NO.Fe(II) EDTA (Adduct)               (XIII)

In addition, however, some of the iron(II) chelate may react withoxygen, if present in the feed gas, to yield an iron(III) chelate, suchas FE(III) EDTA (ferric ion chelated by ethylenediaminetetraaceticacid). But in regenerator 15, under the conditions hereinbeforespecified, the spent absorbent containing dissolved NO and any iron(III)chelate is regenerated to a form once again containing the activeiron(II) chelate, most probably by the following chemical reactions:

    2NO.Fe(II)EDTA+2HCO.sub.2 H→2Fe(II)EDTA+N.sub.2 +2CO.sub.2 +2H.sub.2 O                                                         (XIV)

    2Fe(III)EDTA+HCO.sub.2 H→2Fe(II)EDTA+CO.sub.2 +2H.sup.+(XV)

The innocuous nitrogen produced in regenerator 15 from the dissolved NOis removed as a component of the product gas stream discharged throughvent valve 17.

Thus, in this embodiment of the invention, a single absorbent isutilized to remove both SO_(x) and NO_(x) components, and the spentabsorbent is regenerated by contact in the presence of added formate ionwith a tertiary amine-containing substance as described hereinbefore.One of the highly beneficial features of the invention, therefore, whenboth SO_(x) and NO components must be removed, is that an aqueousabsorbent comprising a formate compound and an iron(II) chelate isuseful for simultaneously removing both SO_(x) and NO in an absorptionzone and, when spent, is capable of being regenerated in a singleregeneration zone.

Another highly beneficial feature of the invention when NO_(x) removalis desired is that any iron(III) chelate formed in the absorber byreaction with oxygen and any iron(III) chelate deliberately added to theabsorbent are converted to iron(II) chelate in regenerator 15 so as toinsure the continuous removal of NO from the feed gas. Iron(III) chelateis not active for removing NO, and accordingly, when NO is to beremoved, iron(III) chelate must be reduced to iron(II) chelate inregenerator 15. A highly convenient visual method for determining if theregenerated absorbent is sufficiently regenerated for purposes of NOabsorption is by the color of the regenerated absorbent. Since iron(II)chelates are almost colorless, while most iron(III) chelates exhibitnoticeable color, usually a brownish-red color in acidic media, theregenerated absorbent from separator 18 recovered in line 19 must eitherbe almost colorless or exhibit a lighter color than the spent absorbentin line 5. Most usually, the regenerated absorbent will evince somecolor, usually a pale amber color, since the conversion of ferricchelate to ferrous chelate is usually not 100% complete. Thus, thepreferred method of operation with iron-chelate-containing absorbentsconverts a spent absorbent of a relatively dark color to a regeneratedabsorbent exhibiting only a tinge of color, and the difference in colorof the two liquids provides a quick indication that the regeneratedabsorbent is active for absorbing NO.

When the process of the invention is utilized to treat a stack gashaving essentially all of its NO_(x) in the form of NO, the NO_(x)removal is such that the purified gas removed from absorber 2 carries,on a mass per hour basis, less than 15% of the amount of NO_(x) carriedin the stack gas. Essentially all NO_(x) absorbed in the absorbentsolution is converted in regenerator 15 to nitrogen and recovered withthe product gas. No significant concentrations of ammonium ion, forexample, have ever been discovered to be present in the absorbentleaving the regenerator. Thus, the concentration of nitrogen componentsin the circulating absorbent does not increase in the manner of sulfateion as hereinbefore described.

In view of the foregoing, it should be apparent that the invention mayeasily be modified so as to be useful for removing NO from a feed gascontaining NO but no SO_(x) compounds, e.g., a stack gas produced fromburning sulfur-free natural gas as fuel. In this modification, it isonly necessary to provide a regenerated absorbent in line 3 containingsufficient of an iron(II) chelate to remove the NO in the feed gas.Spent absorbent is then regenerated in regenerator 15 with sufficientadded formate ion from a source such as formic acid added through line14. Regeneration in regenerator 15 is accomplished, for example, bycontact with formate ion-exchanged Amberlyst A-21 resin in a manner ashereinbefore described for removing either SO_(x) alone or SO_(x) plusNO_(x) compounds.

The following Examples are provided to illustrate the preferredembodiment of the invention and to provide data relating to theperformance of the catalyst for reducing dissolved SO_(x) and NO_(x)components in regenerator 15. The Examples are not provided to limit theinvention, the scope of which is defined by the claims.

EXAMPLE I

A simulated stack gas stream having the composition shown in Table IIwas passed at a pressure slightly greater than atmospheric into alaboratory-sized absorber having a height of 3 feet and containing 1/4inch ceramic Berl saddles at the rate of 1000 scc/min (scc referringherein to the calculated volumetric gas rate at 1 atm. and 60° F.). Theabsorber was operated at a temperature of about 130° F., and thesimulated stack gas was passed upwardly in countercurrent flow with anaqueous liquid absorbent. The absorbent initially had a pH of 4.0 andcomprised monohydrated ferric formate (Fe(CO₂ H)₃.H₂ O) in a 0.18 molarconcentration, sodium HEDTA in a 0.32 molar concentration, sodiumformate in a 1.0 molar concentration, and formic acid in a 1.0 molarconcentration.

Spent absorbent recovered from the absorber was passed upwardly througha regenerator comprising a stirred reactor containing 200 grams ofAmberlyst A-21 weakly basic anion exchange resin (53 wt.% moisture). Theresin had been ion-exchanged to contain formate ions at the ion exchangesites by first immersing the resin in an aqueous solution comprising 90wt.% formic acid and then heating the resulting slurry to 208° F. in anitrogen atmosphere for one hour. After the slurry was separated intosolid particles of resin and an acidic liquid, the resin was washedthree times with distilled water and charged to the reactor.

The volume of the reactor not occupied by the resin was approximately2.0 liters. The operating conditions maintained in the regenerator were285° F. and 63 psia. Concentrated formic acid (98 wt.%) was continuallyadded to the regenerator at a rate of 2.12 ml/hr. The space velocity ofliquid passing through the regenerator vessel, based on the volume ofmoist resin, was 0.38 LHSV.

A fluid mixture of gas and regenerated absorbent was produced in theregenerator and passed to a suitable gas-liquid separator. A product gaswas recovered from the gas liquid separator at a rate of 37.25 scc/minwhile regenerated absorbent was recovered and recycled at a rate of 110ml/hr to the absorber. Water was added as necessary to the recycledabsorbent to maintain a liquid circulation rate of 110 ml/hr.

After operating in the foregoing manner for approximately one week (bywhich time steady state conditions were obtained), a sample of thepurified gas recovered from the absorber at a rate of 997.8 scc/min wasanalyzed, and the results are shown in Table II. Approximately 99% ofthe SO₂ and approximately 89% of the NO in the stack gas feed wereremoved. In addition, samples of the absorbent solution were analyzed,and it was found that the absorbent had a pH of about 4.2 as it enteredthe absorber and about 3.8 when it left. The steady state sulfiteconcentration of the absorbent entering the regenerator was 3.33 g/l(calculated as SO₃ ⁻²) and less than 0.1 g/l while leaving. Samples ofthe absorbent solution taken at various times throughout the week weresubjected to analysis for sulfate concentration. The analysis showedthat the sulfate concentration increased at a substantially stead rateof 0.26 g/l/day (calculated as SO₄ ⁻²).

A sample of the product gas stream recovered from the gas-liquidseparator was chemically analyzed and found to contain the componentsshown in Table II in the concentrations therein shown. These dataindicate much higher than expected recoveries of CO₂ and H₂ from thegas-liquid separator. It was therefore determined that much of theformic acid introduced into the regenerator was decomposing therein andforming H₂ and CO₂. Accordingly, the formic acid feed rate of 2.12 ml/hrwas far in excess of that required in the process, a rate of 0.82 inl/hr being more appropriate. However, the fact that H₂ S was present inthe product gas stream in a concentration of 4.78% is highlysignificant. From the data in Table II and the volumetric gas rates setforth hereinbefore, it was determined that SO₂ was absorbed in theabsorbent at a rate of 1.98 scc/min and recovered as H₂ S at a rate of1.782 scc/min, with the difference largely accumulating in the absorbentin the form of sulfate. Thus the process proved highly efficient for theremoval of SO₂ from a gas stream and conversion of H₂ S. Similarly,since no nitrogen oxides were found in the product gas recovered fromthe gas-liquid separator, the process was shown to be highly effectivefor the removal of NO and conversion to an innocuous form.

                  TABLE II                                                        ______________________________________                                        GAS STREAM COMPOSITIONS                                                       Gas         Simulated   Purified Product                                      Component   Stack Gas   Gas      Gas                                          ______________________________________                                        SO.sub.2, vppm                                                                            2000        20       21                                           NO, vppm    500         55.1     --                                           O.sub.2, vol. %                                                                           3.4         3.41     --                                           CO.sub.2, vol. %                                                                          14.0        13.93    57.75                                        N.sub.2, vol. %                                                                           82.35       82.65    0.36                                         H.sub.2 S, vol. %                                                                         --          --       4.78                                         H.sub.2, vol. %                                                                           --          --       37.06                                        CH.sub.3 SH, vppm                                                                         --          --       450                                          COS, vppm   --          --       29                                           ______________________________________                                    

EXAMPLE II

A series of experiments were run in a manner similar to that describedin Example I to determine the efficiency of the conversion of dissolvedSO₂ components entering the regenerator as a function of operatingtemperature and space velocity. The main differences in theseexperiments that contrast with the Example I experiment are as follows:(1) several runs were performed utilizing a space velocity of absorbentpassing through the regenerator of 0.19 LHSV as well as the 0.38 LHSV ofExample I, (2) operating temperatures were varied in the range of about200° to 290° F., (3) the feed gas to the absorber contained no NO_(x)components and therefore no iron chelates were present in the absorbentsolution, and (4) the absorbent solution consisted essentially of a 1.0molar solution of sodium formate buffered to a pH in the range of 4.0 to4.7 with formic acid. The data collected from the experiment ispresented in the semi-log plot shown in FIG. 2. These data establishthat high conversions to H₂ S of dissolved SO₂ in the absorbent areobtained at temperatures above about 260° F. while significantconversions are obtained at temperatures as low as about 240° F.

Although the invention has been described in conjunction with specificembodiments and examples thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. For example, many reduciblesulfoxy anions besides those shown hereinbefore may be converted tohydrogen sulfide under the conditions maintained in regenerator 15. Forexample, polythionates may be converted according to a chemical reactionsuch as:

    S.sub.2 O.sub.6.sup.= +8HCO.sub.2 H→2H.sub.2 S+8CO.sub.2 +6H.sub.2 O (XVI)

A specifically contemplated aqueous solution which may be treated underthe conditions of regenerator 15 is Wackenroder's solution, an aqueoussolution found during an aqueous Claus reaction. Accordingly, it isintended to embrace this and all such alternatives, modifications, andvariations as fall within the spirit and scope of the appended claims.

I claim:
 1. A process for treating a feed aqueous solution containing one or more anions selected from the group consisting of sulfite, bisulfite, thiosulfite, and polythionate ions, which process comprises (1) contacting, in the presence of formate ion, said feed aqueous solution with a water-insoluble, solid substance containing a tertiary amine functional group under conditions, including an elevated temperature above about 270° F., such that at least 50 percent of said anions are converted to hydrogen sulfide and (2) separating said hydrogen sulfide from a product aqueous solution of reduced sulfur-containing anions content.
 2. A process as defined in claim 1 wherein said solid substance is a catalyst consisting essentially of a water-insoluble, organic ion exchange resin containing tertiary amine functional groups but essentially no primary or secondary amine functional groups, said resin comprising a copolymer of styrene or substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity and containing formate ion in at least 50 percent of the anionic ion exchange sites.
 3. A process as defined in claim 2 wherein said resin component is 2-methyl-5-vinylpyridine.
 4. A process as defined in claim 1 wherein said feed aqueous solution contains sulfite anion, said elevated temperature is between about 270° and 350° F. a substantial proportion.
 5. A process as defined in claim 4 wherein said solid substance is a catalyst comprising a water-insoluble, organic ion exchange resin containing tertiary amine functional groups but essentially no primary or secondary amine functional groups, said resin comprising a copolymer of styrene or substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity and containing formate ion in at least 50 percent of the anionic ion exchange sites.
 6. A process as defined in claim 4 wherein said solid substance is a catalyst consisting essentially of a water-insoluble, organic ion exchange resin containing tertiary amine functional groups, said resin comprising a copolymer of styrene or a substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity, essentially all of which exchange capacity resides in said tertiary amine functional groups, and said resin further containing formate ion in at least 50 percent of the anionic ion exchange sites.
 7. A process as defined in claim 6 wherein said resin component is 4-vinylpyridine.
 8. A process as defined in claim 2, 7, or 3 wherein essentially all of said anionic ion exchange capacity is satisfied with formate ions.
 9. A process for treating a feed aqueous solution containing sulfite anions, which process comprises (1) contacting, in the presence of formate ions, said feed aqueous solution with a resin under conditions, including a temperature above about 270° F., such that at least 50 percent of said sulfite anions are converted to hydrogen sulfide, said resin being a water-insoluble, anionic, ion exchange resin containing weakly basic tertiary amine functional groups and containing formate ion in at least some of the ion exchange sites and (2) separating said hydrogen sulfide from a product aqueous solution of reduced sulfite anion content.
 10. A process as defined in claim 9 wherein formate ions are present in at least 50 percent of the ion exchange sites of the resin.
 11. A process as defined in claim 10 wherein essentially all of the ion exchange capacity of the resin resides in said tertiary amine functional groups.
 12. A process as defined in claim 11 wherein said contacting in step (1) is at a temperature between about 270° and 290° F.
 13. A process as defined in claim 9, 10, 11, or 12 wherein said resin is an organic resin of macroreticular structure comprising a copolymer of tertiary amine-substituted styrene and divinylbenzene.
 14. A process as defined in claim 9, 10, 11, or 12 wherein said resin is an organic resin of macroreticular structure comprising a copolymer of a vinylpyridine and divinylbenzene.
 15. A process as defined in claim 14 wherein said vinylpyridine is selected from the group consisting of 4-vinylpyridine, 2-methyl-5-vinylpyridine and 2-vinylpyridine.
 16. A process as defined in claim 1 wherein said solid substance is a catalyst comprising a water-insoluble, organic ion exchange resin containing tertiary amine functional groups, said resin comprising a copolymer of styrene or a substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity, essentially all of which exchange capacity resides in said tertiary amine functional groups, and said resin further containing formate ion in at least 50 percent of the anionic ion exchange sites.
 17. A process as defined in claim 9, 10, 11, 12, 16, 5, 6, 2, 7, or 3 wherein said resin is relatively free of quaternary amines.
 18. A process as defined in claim 17 wherein said anionic ion exchange capacity is essentially completely satisfied with formate ions.
 19. A process as defined in claim 16 or 5 wherein said resin component is 4-vinylpyridine.
 20. A process as defined in claim 19 wherein essentially all of said anionic ion exchange capacity is satisfied with formate ions.
 21. A process as defined in claim 16 or 5 wherein said resin component is 2-methyl-5-vinylpyridine.
 22. A process as defined in claim 21 wherein essentially all of said anionic ion exchange capacity is satisfied with formate ions.
 23. A process as defined in claim 16 or 5 wherein said resin is of macroreticular structure.
 24. A process as defined in claim 16 or 5 wherein essentially all of said anionic ion exchange capacity is satisfied with formate ions.
 25. A process as defined in claim 16, 5, 6, 2, 3, or 7 wherein said catalyst has been prepared by a method comprising the step of reacting, in an aqueous medium, divinylbenzene, styrene or a substituted styrene, a vinylpyridine or a substituted vinylpyridine, a fatty acid soap, a tertmercaptan, and a compound selected from the group consisting of potassium persulfate or benzoyl peroxide and contacting the reaction product of said reacting with a source of formate ion.
 26. A process as defined in claim 25 wherein said resin is relatively free of quaternary amines.
 27. A process as defined in claim 26 wherein said anionic ion exchange capacity is essentially completely satisfied with formate ions.
 28. A process as defined in claim 16, 5, 6, 2, 3, or 7 wherein said catalyst has been prepared by a method comprising the step of reacting, in an aqueous medium, divinylbenzene, styrene, 4-vinylpyridine, a fatty acid soap, a tert-mercaptan, and benzoyl peroxide and contacting the reaction product of said reacting with a source of formate ion.
 29. A process as defined in claim 16, 5, 6, 2, 3, or 7 wherein said catalyst has been prepared by a method comprising the step of reacting, in an aqueous medium, divinylbenzene, styrene, 2-methyl-5-vinylpyridine, a fatty acid soap, a tert-mercaptan, and potassium persulfate and contacting the reaction product of said reacting with a source of formate ion.
 30. A process for treating a feed aqueous solution containing one or more anions selected from the group consisting of sulfite, bisulfite, thiosulfate, and polythionate ions, which process comprises (1) contacting, in the presence of formate ion, said feed aqueous solution with a water-insoluble, solid substance containing a tertiary amine functional group under conditions, including an elevated temperature above about 270° F., such that more of said anions are converted to hydrogen sulfide than to elemental sulfur and (2) separating said hydrogen sulfide from a product aqueous solution of reduced sulfur-containing anions content.
 31. A process as defined in claim 30 wherein said feed aqueous solution contains sulfite anion, said elevated temperature is between about 270° and 350° F., and a substantial proportion of said sulfite anions are converted to hydrogen sulfide.
 32. A process as defined in claim 31 wherein said solid substance is a catalyst comprising a water-insoluble, organic ion exchange resin containing tertiary amine functional groups but essentially no primary or secondary amine functional groups, said resin comprising a copolymer of styrene or substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity and containing formate ion in at least 50 percent of the anionic ion exchange sites.
 33. A process as defined in claim 32 wherein said resin component is 2-methyl-5-vinylpyridine.
 34. A process as defined in claim 32 wherein essentially all of said anionic ion exchange capacity is satisfied with formate ions.
 35. A process as defined in claim 30 wherein said solid substance is a catalyst comprising a water-insoluble, organic ion exchange resin containing tertiary amine functional groups, said resin comprising a copolymer of styrene or a substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity, essentially all of which exchange capacity resides in said tertiary amine functional groups, and said resin further containing formate ion in at least 50 percent of the anionic ion exchange sites.
 36. A process as defined in claim 35 wherein said resin component is 4-vinylpyridine.
 37. A process as defined in claim 35 wherein said resin is of macroreticular structure.
 38. A process for treating a feed aqueous solution containing sulfite anions, which process comprises (1) contacting, in the presence of formate ions, said feed aqueous solution with a solid resin under conditions, including a temperature above about 270° F., such that more of said sulfite anions are converted to hydrogen sulfide than to elemental sulfur, said resin being a water-insoluble, anionic, ion exchange resin containing weakly basic tertiary amine functional groups and containing formate ion in at least some of the ion exchange sites and (2) separating said hydrogen sulfide from a product aqueous solution of reduced sulfite anion content.
 39. A process as defined in claim 38 wherein formate ions are present in at least 50 percent of the ion exchange sites of the resin.
 40. A process as defined in claim 39 wherein essentially all of the ion exchange capacity of the resin resides in said tertiary amine functional groups.
 41. A process as defined in claim 38 wherein said contacting in step (1) is at a temperature between about 270° and 290° F. and a substantial proportion of said sulfite anions are converted to hydrogen sulfide.
 42. A process as defined in claim 41 wherein said resin is an organic resin of macroreticular structure comprising a copolymer of a vinylpyridine and divinylbenzene.
 43. A process as defined in claim 38 wherein said resin is an organic resin of macroreticular structure comprising a copolymer of tertiary amine-substituted styrene and divinylbenzene.
 44. A process for treating a feed aqueous solution containing one or more anions selected from the group consisting of sulfite, bisulfite, thiosulfate, and polythionate ions, which process comprises (1) contacting, in the presence of formate ion, said feed aqueous solution with a water-insoluble, solid substance containing a tertiary amine functional group under conditions, including an elevated temperature between about 280° and 300° F., such that at least 50 percent of said anions are converted to hydrogen sulfide and (2) separating said hydrogen sulfide from a product aqueous solution of reduced sulfur-containing anions content.
 45. A process as defined in claim 44 wherein said feed aqueous solution contains sulfite anion.
 46. A process as defined in claim 45 wherein said solid substance is a catalyst comprising a water-insoluble, organic ion exchange resin containing tertiary amine functional groups but essentially no primary or secondary amine functional groups, said resin comprising a copolymer of styrene or substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity and containing formate ion in at least 50 percent of the anionic ion exchange sites.
 47. A process as defined in claim 46 wherein said resin component is 4-vinylpyridine.
 48. A process as defined in claim 44 wherein said solid substance is a catalyst comprising a water-insoluble, organic ion exchange resin containing tertiary amine functional groups, said resin comprising a copolymer of styrene or a substituted styrene, divinylbenzene, and a resin component selected from the group consisting of the vinylpyridines and the substituted vinylpyridines, said resin having anionic ion exchange capacity, essentially all of which exchange capacity resides in said tertiary amine functional groups, and said resin further containing formate ion in at least 50 percent of the anionic ion exchange sites.
 49. A process as defined in claim 48 wherein said resin component is 2-methyl-5-vinylpyridine.
 50. A process as defined in claim 48 wherein essentially all of said anionic ion exchange capacity is satisified with formate ions.
 51. A process as defined in claim 46 wherein said resin is of macroreticular structure.
 52. A process for treating a feed aqueous solution containing sulfite anions, which process comprises (1) contacting, in the presence of formate ions, said feed aqueous solution with a solid resin under conditions, including a temperature between about 280° and 300° F., such that more of said sulfite anions are converted to hydrogen sulfide than to elemental sulfur, said resin being a water-insoluble, anionic, ion exchange resin containing weakly basic tertiary amine functional groups and containing formate ion in at least some of the ion exchange sites and (2) separating said hydrogen sulfide from a product aqueous solution of reduced sulfite anion content.
 53. A process as defined in claim 52 wherein formate ions are present in at least 50 percent of the ion exchange sites of the resin.
 54. A process as defined in claim 53 wherein essentially all of the ion exchange capacity of the resin resides in said tertiary amine functional groups.
 55. A process as defined in claim 52 wherein said resin is an organic resin of macroreticular structure comprising a copolymer of tertiary amine-substituted styrene and divinylbenzene.
 56. A process as defined in claim 52 wherein said resin is an organic resin of macroreticular structure comprising a copolymer of a vinylpyridine and divinylbenzene. 